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Cicada on leaf.

Cicada Cycles Benefit Valley Region

Time-Keeping Insects Help Forests, Wildlife and People

Buzzing, whining, beautiful – the cicadas are coming.

But have no fear.

These red-eyed bumbling insects aren’t dangerous to people, pets or crops.

In fact, they’re tasty to wildlife and fish, who feast on their crunchy exoskeletons by the millions.

And they’re part of a natural cycle that’s been going on for millennia.

Many Valley residents will see the southern Brood XIX for about a month between April and June. Residents of northern states will spy members of cicada Brood XIII.

It’s the first time these particular broods have crooned ballads in the same year since 1803.

And if people aren’t in the path this year, just wait. Members of different broods will hatch in different parts of the Valley in upcoming spring seasons.

“(Cicada hatching) connects us to the natural world,” Jesse Troxler, Tennessee Valley Authority terrestrial zoologist, said. “Wherever you're located, you're going to get a different emergence, and you may not see them again for a long time.”

Cicada on a plant stem.

Cicada Cycles

North Americans regularly hear the warm-weather whine of two types of cicadas – annual and periodical.

Annual cicadas sing every year during the sultry dog days of summer, when the Dog Star Sirius rises in the sky.

Periodical cicadas – the kind that makes up the different broods that emerge in cycles – live on their own quirky timelines.

In TVA’s seven-state service area, four species of the southern 13-year cicadas emerge. In northern states, broods made of three species of 17-year cicadas are more common.

Why the different timelines in different regions?

Some scientists think past glaciation set these cycles in motion.

Still others think cicadas are in stealth mode – they emerge after long, prime-number intervals so they don’t sync up with other animals’ life cycles.

And these cyclical cicadas count on sheer numbers, too. Raccoons, bass, crows and others can gorge themselves on the nutty-flavored nuggets and still leave plenty of surviving insects to breed.

Whatever the reasons, this season of emerging periodical cicadas is special.

“Cicadas are such an awesome event in nature, especially this year,” TVA terrestrial zoologist Maria Aguirre said. “They spend 99% of their life underground and just come out for just a little bit to mate.”

This year, the broods won’t overlap much. Only Illinois residents will be able to hear both the 13- and 17-year cicadas a short drive apart.

Even side by side, though, the seven cicada species look almost alike. Some species, like the Cassini group – also called dwarf cicadas – have smaller bodies.

And some sound different. The Pharaoh gets its name from its long croak, "Phaaaaraoh," Troxler explained.

Aguirre had some advice for people wary of insects.

“I think they should see (the emergence) as natural and interesting,” she said. “If you don't like bugs, I get it. But just don't bother them. They won't bother you. They'll just be a little loud, but please don't spray them. Cicadas are just really cool.”

Cicada shedding on tree.

Tuned Into Timing

Cicadas know the weather and calendar. They claw their way to daylight when the soil hits exactly 64 degrees at 8 inches underground – on exactly the 13th or 17th year after they hatched.

“They'll tunnel to the surface and make holes there,” Troxler said. “Then everybody's getting ready – waiting, waiting, waiting. Once they get a good rain, it's go time.”

Go time means the wingless larvae carefully climb with hooked feet up a rough vertical surface, such as a tree or brick wall, to molt the tan husk of their outer skin.

“They'll split that exoskeleton and their wings will emerge,” Troxler said. “They'll unfold them (and) pump fluid into those wings. They just inflate them like a bicycle tire. They'll be there for a couple of hours drying out before they can actually fly.”

Then the weeks-long frenzy begins. Males create a whirring, overlapping chorus, filling the forest with a love song to females.

“The male has clapper organs called tymbals on its abdomen, and it's mainly hollow on the inside, so that the sound can reverberate,” Aguirre said.

Females click back and, after their eggs are fertilized, climb to the tender tips of branches.

“Then they'll saw a little slit into the twig and deposit their eggs inside,” Troxler said. “It doesn't affect mature trees. They're strong enough to take that hit.”

Still, he advised, homeowners may want to cover tiny trees with nursery cloth.

In the big picture, cicadas help forests, Shannon O’Quinn, TVA senior water resources specialist and avid fly fisher, said. “(The) million cicadas that were in that tree, they all die and fertilize that tree afterwards.”

The event isn’t over even after the singing has stopped.

“In July, August, you can see the eggs, hatching … from the tree branches and falling onto the ground,” Aguirre said. “All the little eggs coming down … look misty.”

Baby cicadas dig down into the soil, bidding the sun goodbye for another 13 to 17 years, and the cycle begins again.

In upcoming years, members of different periodical broods will hatch throughout the Valley, singing their songs of spring.


Awesome Animals

Cicadas are best known for their timing, but they play a bigger role in ecosystems, too.

Besides providing food for wildlife and trees, cicadas dig tunnels that let air and water into the soil. Birds even leave or adjust when they sing, and at what frequency, so as not to compete with the noisy insects.

And eager fly fishers await cicadas, too.

Avid casters tie flies to mimic the bugs’ bulky bodies and colors, then float fishing lines over shallow waters of reservoirs and tailwaters to reel in trout, bass and invasive orange carp.

Across the Valley and the continent, cicadas’ songs tell of long-term biodiversity.

“If (the cicadas) are there, that means that that ecosystem has been pretty stable,” Aguirre said.

That’s why people have the best chance to hear and see them in older parks and wooded backyards.

“You're looking for an area with lots of mature deciduous trees – the kind that lose their leaves in the fall,” Troxler said. “The bigger, more mature trees … the more chance you have of cicadas. They go back 13 years, 26 years, 39 years.”

Cicadas seem common now, but people shouldn’t take these noisy insects for granted.

“They're extremely vulnerable to development,” David Harrell, TVA’s floating cabins program manager, said. “(If) a strip mall comes in and now (the ground) is pavement, well, (the cicadas) are done for. They will not occur at that spot again."

“And there's actually been documentation of a couple of broods that no longer exist because of that,” O’Quinn said.

That’s why noticing cicadas and seeing people as connected to the region’s ecosystem matters.

People can become citizen scientists and add cicada sightings to tracking maps this spring, as well as look forward to upcoming emergences in their area.

“We all spend most of our time inside these days,” Troxler said. “The more that you’re aware of what's going on outside of your walls, the more you'll take care of it.”

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Learn more about TVA’s work in biodiversity conservation at the Biodiversity page.

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Cicada facts

  • Harmless to humans
  • Tasty, protein-rich snacks for many land animals, birds and fish
  • Funny, clumsy flyers
  • Offer amazing fly-fishing opportunities
  • Emerge in exact 13- or 17-year cycles
  • Unable to bite or sting
  • Unrelated to crop-eating locusts or grasshoppers
  • Not harmful to gardens, crops or mature forests
  • Not poisonous
  • Not aggressive to humans or animals
  • They aerate and add nutrients to the soil